Multi-channel inverter systems including coupled inductors

ABSTRACT

A system comprises a plurality of inverter units having inputs connected to a power source and a coupled inductor comprising a plurality of windings and coupled between the plurality of inverter units and an output filter, wherein each winding of the plurality of windings has a first terminal connected to an output of a corresponding inverter unit and second terminals of the plurality of windings are connected together.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. application Ser. No.14/933,723, filed on Nov. 5, 2015, entitled “Multi-Channel InverterSystems”, now U.S. Pat. No. 9,923,485, which application is herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an apparatus, system and method forload balancing among multiple inverter units connected in parallel, and,in particular embodiments, to an apparatus, system and method for loadbalancing through a coupled inductor placed between parallel operatedinverter units and an output filter.

BACKGROUND

Renewable energy sources include solar energy, wind power, tidal waveenergy and the like. A solar power conversion system may include aplurality of solar panels connected in series or in parallel. The outputof the solar panels may generate a variable dc voltage depending on avariety of factors such as time of day, location and sun trackingability. In order to regulate the output of the solar panels, the outputof the solar panels may be coupled to a dc/dc converter so as to achievea regulated output voltage at the output of the dc/dc converter. Inaddition, the solar panels may be connected with a backup battery systemthrough a battery charge control apparatus. During the day, the backupbattery is charged through the output of the solar panels. When thepower utility fails or the solar panels are an off-grid power system,the backup battery provides electricity to the loads coupled to thesolar panels.

Since the majority of applications may be designed to run on 120 voltsac power, a solar inverter is employed to convert the variable dc outputof the photovoltaic modules to a 120 volts ac power source. In order toattenuate undesirable harmonics, a plurality of magnetic devices may becoupled between the solar inverter and the ac power source.

A magnetic device typically includes a magnetic core formed of suitablemagnetic materials such as ferrite, powder iron and/or the like. Themagnetic device may further include a conductive winding or a pluralityof conductive windings. The windings and the current flowing through thewindings may generate a magnetic field, which is also known as magneticflux. In a normal design, the magnetic core usually has a relativelyhigh permeability in comparison with the surrounding medium (e.g., air).As a result, the magnetic flux is confined with the magnetic core, whichforms a closed flux path. The magnetic flux provides a medium forstoring, transferring or releasing electromagnetic energy.

Coupled inductors are widely used in the power electronics industry. Acoupled inductor may comprise two windings magnetically coupled to eachother. The two coupled windings may be wound on a same magnetic core(e.g., a toroid core). The first winding generates a first magneticforce, which drives a first magnetic field or flux. The flux generatedby the first winding is confined with the magnetic core, which forms aclosed flux path. Likewise, the second winding generates a secondmagnetic force, which drives a second magnetic field, which is confinedwith the magnetic core. The magnetic material of the magnetic core of acoupled inductor may be of a magnetic permeability greater than that ofa surrounding medium (e.g., air). However, the coupling between twowindings of the coupled inductor is not perfect. There may be a leakagepath between the winding and the surrounding medium having a lowermagnetic permeability.

SUMMARY

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention which provide a multi-channel inverter.

In accordance with an embodiment, a system comprises a plurality ofinverter units having inputs connected to a power source and a coupledinductor comprising a plurality of windings and coupled between theplurality of inverter units and an output filter, wherein each windingof the plurality of windings has a first terminal connected to an outputof a corresponding inverter unit and second terminals of the pluralityof windings are connected together.

In accordance with another embodiment, a system comprises a firstinverter connected to a power source, a second inverter connected to thepower source, wherein the second inverter is configured to operate witha first phase shift from the first inverter, a third inverter connectedto the power source, wherein the third inverter is configured to operatewith a second phase shift from the second inverter, a coupled inductorhaving a first input, a second input and a third input connected to thefirst inverter, the second inverter and the third inverter respectivelyand an output filter coupled to an output of the coupled inductor.

In accordance with yet another embodiment, a three-phase inverter systemcomprises a first phase comprising a plurality of first invertersconnected together through a first coupled inductor, a second phasecomprising a plurality of second inverters connected together through asecond coupled inductor, a third phase comprising a plurality of thirdinverters connected together through a third coupled inductor and afourth coupled inductor having a first winding connected to the firstcoupled inductor, a second winding connected to the second coupledinductor and a third winding connected to the third coupled inductor.

An advantage of an embodiment of the present invention is amulti-channel inverter providing higher efficiency as well as lowertotal harmonic distortion (THD).

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a multi-channel inverter systemhaving a coupled inductor in accordance with various embodiments of thepresent disclosure;

FIG. 2 illustrates a schematic diagram of a first implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure;

FIG. 3 illustrates a schematic diagram of a second implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure;

FIG. 4 illustrates schematic diagrams of a third implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure;

FIG. 5 illustrates schematic diagrams of a fourth implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure;

FIG. 6 illustrates an implementation of the coupled inductor shown inFIG. 1 in accordance with various embodiments of the present disclosure;

FIG. 7 illustrates a schematic diagram of a three-phase systemcomprising the multi-channel inverter system shown in FIG. 1 inaccordance with various embodiments of the present disclosure; and

FIG. 8 illustrates a multi-level waveform of the multi-channel invertersystem shown in FIG. 2 in accordance with various embodiments of thepresent disclosure.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely a coupled inductor structurefor connecting a plurality of inverter units of a phase of a dc/ac powersystem. Furthermore, the coupled inductor structure may be employed toconnect three phases of the dc/ac power system. The invention may alsobe applied, however, to a variety of dc/ac power systems. Hereinafter,various embodiments will be explained in detail with reference to theaccompanying drawings.

FIG. 1 illustrates a block diagram of a multi-channel inverter systemhaving a coupled inductor in accordance with various embodiments of thepresent disclosure. The multi-channel inverter system 100 comprises aninput dc power source 101, a multi-channel inverter 102, a coupledinductor 104, an output filter 106 and an ac power source 107. In someembodiments, the multi-channel inverter 102 may comprises a plurality ofpower processing channels. Each channel may be implemented as a dc/acinverter. The channels in the multi-channel inverter 102 are connectedin parallel through the coupled inductor 104. More particularly, theinputs of each channel are connected to the input dc power source 101and the output of each channel is connected to an input of the outputfilter 106 through a winding of the coupled inductor 104. Equal currentsharing may be achieved through the coupled inductor 104. Throughout thedescription, the channels of the multi-channel inverter 102 may bealternatively referred to as the inverter units of the multi-channelinverter 102.

Each inverter unit of the multi-channel inverter 102 inverts a dcwaveform received from the input dc power source 101 to an ac waveform.In some embodiments, each inverter unit may comprise a plurality ofswitching elements such as insulated gate bipolar transistor (IGBT)devices. Alternatively, each inverter unit may include other types ofcontrollable devices such as metal oxide semiconductor field effecttransistor (MOSFET) devices, bipolar junction transistor (BJT) devices,super junction transistor (SJT) devices, bipolar transistors and/or thelike. The detailed operation and structure of the inverter units of themulti-channel inverter 102 will be described below with respect to FIG.2.

In some embodiments, each channel is configured to invert a dc waveforminto an ac waveform with a phase shift. The phase shift of each powerprocessing channel is equal to 360 degrees divided by N, where N is theorder of the most significant harmonic of the multi-channel invertersystem 100. In some embodiments, N is an odd integer such as 5, 7 andthe like.

The coupled inductor 104 may comprise a plurality of windingsmagnetically coupled to each other. In some embodiments, the pluralityof windings may be negatively coupled to each other. The plurality ofcoupled windings may be wound on a same magnetic core (e.g., a toroidcore). First terminals of the plurality of coupled windings areconnected to their respective channels and second terminals of theplurality of coupled windings are connected together and furtherconnected to the input of the output filter 106. Furthermore, thecoupling among the plurality of windings may generate leakage magneticflux. In an equivalent circuit of a coupled inductor, the leakagemagnetic flux is replaced by a leakage inductance. Such a leakageinductance may be employed to replace some inductive elements of theoutput filter 106. The structure of the coupled inductor 104 will bedescribed in detail below with respect to FIGS. 2-5.

The output filter 106 may comprise a plurality of inductive andcapacitive elements. In some embodiments, the inductive and capacitiveelements may form an L-C filter or a plurality of L-C filters connectedin cascade. The inductive elements (e.g., inductors) provide highimpedance to prevent high frequency noise from flowing out of themulti-channel inverter system 100. At the same time, the capacitiveelements (e.g., capacitors) shunt the input of the ac power source 107and provide a low impedance channel for the high frequency noisegenerated from the inverter units. The detailed structure of the outputfilter 106 will be described below with respect to FIGS. 2-5.

FIG. 2 illustrates a schematic diagram of a first implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure. An input dc voltage source PV1 iscoupled to the inputs of the multi-channel inverter 102. In order tofilter input noise, input capacitors (e.g., C1 and C2) are connectedbetween the two output terminals of the input dc voltage source PV1.More particularly, the input capacitors may comprise a first inputcapacitor C1 and a second input capacitor C2. The first input capacitorC1 and the second input capacitor C2 are connected in series and furthercoupled between the two terminals of the input dc voltage source PV1.The common node of the first input capacitor C1 and the second inputcapacitor C2 is connected to a neural point as shown in FIG. 2. In someembodiments, the neural point is the ground of the multi-channelinverter system 200.

The multi-channel inverter 102 comprises a first inverter unit 202, asecond inverter unit 204 and a third inverter unit 206. The inputs ofthe inverter units 202, 204 and 206 are connected to the input dcvoltage source PV1. The outputs of the inverter units 202, 204 and 206are connected to the three input terminals of the coupled inductor 104respectively.

The inverter units 202, 204 and 206 shown in FIG. 2 are commonly knownas T-type three-level inverters. It should be noted that while FIG. 2shows the inverter units 202, 204 and 206 are T-type three-level powerinverters, it is within the scope and spirit of the invention for themulti-channel inverter system 200 to comprise other inverters, such as,but no limited to two-level inverters, three-level inverters, resonantinverters, any combinations thereof and/or the like.

In some embodiments, the inverter units 202, 204 and 206 may be of asame structure such as the T-type three-level inverter structure shownin FIG. 2. For simplicity, only the detailed structure of the firstinverter unit 202 will be described below.

The first inverter unit 202 comprises a pair of switching elements S11and S12 connected in series. The common node of the switching elementsS11 and S12 are coupled to ground through an isolation device formed byback-to-back connected switching elements S13 and S14. The back-to-backconnected switching elements S13 and S14 are capable of completelyisolating the common node of the switching elements S11 and S12 fromground. According to some embodiments, switching elements S11, S12, S13and S14 are IGBT or IGBTs connected in parallel, series and anycombinations thereof.

Switching elements S11, S12, S13 and S14 are so controlled that theoutput of the first inverter unit 202 generates a three level waveform.In particular, when switching element S11 is turned on and switchingelement S12 is turned off, the output of the first inverter unit 202generates a positive voltage equal to one half of the voltage of theinput dc voltage source PV1. Likewise, when switching element S12 isturned on and switching element S11 is turned off, the output of thefirst inverter unit 202 generates a negative voltage equal to one halfof the voltage of the input dc voltage source PV1. When both switchingelements S11 and S12 are turned off and switching elements S13 and S14are turned on, the output of the first inverter unit 202 is coupled toground. As such, the output of the first inverter unit 202 generates athree-level voltage waveform. The frequency of the three-level voltagewaveform is approximately 60 HZ in accordance with an embodiment.

Furthermore, the switching elements (e.g., S11, S12, S13 and S14) ofeach inverter unit shown in FIG. 2 are so controlled that there is aphase shift between two output waveforms. For example, by controllingthe timing of the switching elements S11-S14 and the timing of theswitching elements S21-S24, a first phase shift may be placed betweenthe output of the first inverter unit 202 and the second inverter unit204. Likewise, by controlling the timing of the switching elementsS21-S24 and the timing of the switching elements S31-S34, a second phaseshift may be placed between the output of the second inverter unit 204and the third inverter unit 206. Due to the first phase shift and thesecond phase shift, the voltage waveform at the output of the coupledinductor 104 is not a three-level waveform. Instead, the voltagewaveform at the output of the coupled inductor 104 is a multi-levelwaveform. The number of levels of the multi-level waveform may varydepending on the degrees of the phase shifts. In some embodiments, thenumber of levels of the multi-level waveform is equal to seven. Anexample of a seven-level waveform is shown in FIG. 8.

One advantageous feature of having the multi-level waveform (e.g.,waveform D in FIG. 8) of the output of the coupled inductor 104 is thatthe multilevel waveform shown in FIG. 8 follows the sinusoidal waveform.As a result, the harmonic contents are less. For example, by selectingthe phase shift, a dominant harmonic such as the fifth harmonic may beeliminated. Another advantageous feature of having the multi-levelwaveform (e.g., waveform D in FIG. 8) is that the multilevel waveformhelps to simplify the design of the output filter 106. The detailedstructure of the simplified output filter will be described below withrespect to FIGS. 4-5.

It should be noted that the inverter units 202, 204 and 206 and thecoupled inductor 104 shown in FIG. 2 are merely examples, one personskilled in the art will realize that the inverters (e.g., inverter units202-206) as well as its corresponding coupled inductor 104 may beimplemented in many different ways. For example, the multi-channelinverter system 200 may accommodate more than three inverter units.Accordingly, the coupled inductor no may comprise multiple windings,each of which is connected to one output of a corresponding inverterunit.

The coupled inductor 104 comprises three windings. A first winding 212is connected between node A and node D as shown in FIG. 2. A secondwinding 214 is connected between node B and node D. A third winding 216is connected between node C and node D. As shown in FIG. 2, the dot ofeach winding denotes the polarity of the winding. As shown in FIG. 2,the first winding 212 is negatively coupled to the second winding 214with a coupling coefficient of M. Likewise, the second winding 214 isnegatively coupled to the third winding 216 with a coupling coefficientof M. The third winding 216 is negatively coupled to the first winding212 with a coupling coefficient of M. M is a predetermined number. Itmay vary depending on different design needs. The detailedimplementation of these three negatively coupled inductors will bedescribed below with respect to FIG. 6.

In some embodiments, the coupled inductor 104 helps the multi-channelinverter system 200 distribute energy evenly between the first inverterunit 202, the second inverter unit 204 and the third inverter unit 206.In particular, the balance between these three inverter units isdetermined by the magnetizing inductances of the coupled inductor 104.In order to achieve a balanced energy distribution among these threeinverter units, the magnetizing inductances are designed to have a largevalue. As a result, the variations of the magnetizing inductances arerelatively small. Such small variations help the multi-channel invertersystem 200 achieve both static current sharing and dynamic currentsharing.

The output filter 106 may comprise a first inductor L1 and a secondinductor L2 connected in series and further coupled between the coupledinductor 104 and the output ac source. The output filter 106 may furthercomprise a plurality of output capacitors coupled between the commonnode of the first inductor L1 and the second inductor L2, and ground.The plurality of output capacitors are collectively shown as a capacitorC3 in FIG. 1.

The first inductor L1 and the second inductor L2 provide high impedancewhen high frequency noise tries to flow out of the multi-channelinverter system 200. At the same time, the capacitor C3 shunts the inputof the ac source and provides a low impedance channel for the highfrequency noise generated from the multi-channel inverter system 200. Asa result, the high frequency noise of the inverter units 202, 204 and206 may be prevented from passing through the output filter 106.

It should be noted that the inverter topologies, the output filter 106,the input dc power source PV1 and the output ac power source shown inFIG. 2 are provided for illustrative purposes only, and are providedonly as examples of various embodiments. Such examples should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

FIG. 3 illustrates a schematic diagram of a second implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure. The multi-channel inverter system300 shown in FIG. 3 is similar to that shown in FIG. 2 except that thefirst inductor L1 has been replaced by the leakage inductance of thecoupled inductor 104. Since the voltage at the output of the coupledinductor 104 is a multilevel waveform following the sinusoidal waveform,the harmonic content is reduced accordingly. Such a low harmonic contenthelps to simplify the design of the output filter 106. In someembodiments, the inductance of the first inductor L1 may be reduced by90%. In some embodiments, the inductance of the first inductor L1 isequal to about 10 uH. Such a small inductance can be replaced by theleakage inductance of the coupled inductor 104. In other words, thefirst inductor L1 can be integrated into the coupled inductor 104.

The magnetic material of the magnetic core may be of a magneticpermeability greater than that of a surrounding medium (e.g., air).However, the coupling between two inductors of the coupled inductor 104may be not perfect. The coupling between the winding and the surroundingmedium may generate leakage magnetic flux. All three inductors of thecoupled inductor 104 may generate leakage inductance through thecoupling with the surrounding medium such as air. As shown in FIG. 3,the leakage generated by the first inductor winding is defined asL_(lk1); the leakage generated by the second inductor winding is definedas L_(lk2); the leakage generated by the third inductor winding isdefined as L_(lk3). These three leakage inductances can be employed toreplace the first inductor L1 shown in FIG. 2.

FIG. 4 illustrates schematic diagrams of a third implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure. The multi-channel inverter system400 shown in FIG. 4 is similar to that shown in FIG. 2 except that theoutput capacitor C3 has been removed. One drawback of having aninductor-capacitor-inductor (LCL) filter shown in FIG. 2 is the LCLfilter may lead to oscillation in the multi-channel inverter system 200.By increasing the inductance of the output filter (e.g., the inductanceof L1 and/or L2 equal to 30 uH), the capacitor C3 may be removed and themulti-channel inverter system 400 may achieve the same harmonicelimination results (the current THD is equal to 1.8%) as themulti-channel inverter system 200 shown in FIG. 2.

One advantageous feature of removing the output capacitor C3 is thebehavior of the multi-channel inverter system 400 is similar to acurrent source generating a current following a sinusoidal waveform.Such a current source does not have the oscillation issue occurred inthe inverter system having the LCL filter.

FIG. 5 illustrates schematic diagrams of a fourth implementation of themulti-channel inverter system shown in FIG. 1 in accordance with variousembodiments of the present disclosure. The multi-channel inverter system500 shown in FIG. 5 is similar to that shown in FIG. 4 except that thefirst inductor L1 has been replaced by the leakage inductance of thecoupled inductor 104. The mechanism of integrating the first inductor L1into the coupled inductor 104 has been described above in detail withrespect to FIG. 3, and hence is not discussed again to avoid unnecessaryrepetition.

FIG. 6 illustrates an implementation of the coupled inductor shown inFIG. 1 in accordance with various embodiments of the present disclosure.The coupled inductor 104 may be wound around a magnetic core as shown inFIG. 6. In accordance with an embodiment, the magnetic core is made of amagnetic material having high permeability such as ferrite, powder iron,other power suitable materials, any combinations thereof and/or thelike. Furthermore, the magnetic core may be made of suitable alloys suchas silicon steel to further reduce the magnetic losses.

FIG. 6 shows the coupled inductor 104 comprises three winding coils N1,N2 and N3 wound around a magnetic core having three legs. The firstwinding coil N1 is wound around a first leg. The second winding coil N2is wound around a second leg. The third winding coil N3 is wound arounda third leg. As shown in FIG. 6, the second leg is between the first legand the third leg. The input terminals of the coils N1, N2 and N3 areconnected to terminals A, B and C respectively. The output terminals ofthe coils N1, N2 and N3 are connected together and further connected tothe terminal D. It should be noted that coils N1, N2 and N3 are wound inthe same direction. In other words, the magnetic fields generated bywindings N1, N2 and N3 are in the same direction.

FIG. 7 illustrates a schematic diagram of a three-phase systemcomprising the multi-channel inverter system shown in FIG. 1 inaccordance with various embodiments of the present disclosure. Thethree-phase system 700 includes phase A, phase B and phase C. Thestructures of phase B and phase C are similar to that of phase A, andhence are not shown in detail in FIG. 7. In some embodiments, phase Amay be implemented as a multi-channel inverter 702 having three inverterunits. The three inverter units of phase A are connected togetherthrough a first coupled inductor 712. The operating principle of themulti-channel inverter 702 is similar to that of the multi-channelinverter system 200 shown in FIG. 2, and hence is not discussed again toavoid unnecessary repetition.

Phase A, phase B and phase C are connected together through a secondcoupled inductor 714. As shown in FIG. 7, the second coupled inductor714 comprises windings 722, 724 and 726. The windings 722, 724 and 726are connected to loads AC1, AC2 and AC3 respectively through theircorresponding output inductors L11, L12 and L13. The structure of thesecond coupled inductor 714 is similar to that of the first coupledinductor 712, and hence is not discussed again herein.

FIG. 8 illustrates a multi-level waveform of the multi-channel invertersystem shown in FIG. 2 in accordance with various embodiments of thepresent disclosure. FIG. 8 shows four waveforms. The first waveform isgenerated at the output of the first inverter unit 202. The secondwaveform is generated at the output of the second inverter unit 204. Asshown in FIG. 8, there is a phase shift between the first waveform andthe second waveform. The third waveform is generated at the output ofthe third inverter unit 206. As shown in FIG. 8, there is a phase shiftbetween the second waveform and the third waveform. The fourth waveformis generated at the output of the coupled inductor 104. As shown in FIG.8, the fourth waveform has seven levels. The seven-level waveformfollows a sinusoidal waveform. As a result, the harmonic contents areless.

Although embodiments of the present invention and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A system comprising: a plurality of inverterunits having inputs connected to a power source; and a coupled inductorcomprising at least three windings and coupled between the plurality ofinverter units and an output filter, wherein: each winding of the atleast three windings has a first terminal connected to an output of acorresponding inverter unit; second terminals of the at least threewindings are connected together; and the output filter comprises a firstinductor and a second inductor connected in series between the coupledinductor and an output port of the system, and an output capacitorconnected to a common node of the first inductor and the secondinductor.
 2. The system of claim 1, wherein: an inverter unit of theplurality of inverter units comprises a first switch leg and a secondswitch leg, and wherein: the first switch leg comprises a first switchand a second switch connected in series; and the second switch legcomprises two back-to-back connected switches.
 3. The system of claim 2,wherein: the first switch is connected between a first terminal of thepower source and the coupled inductor; the second switch is connectedbetween a second terminal of the power source and the coupled inductor;and the two back-to-back connected switches are connected between acommon node of the first switch and the second switch, and a neutralpoint.
 4. The system of claim 1, wherein: the coupled inductor comprisesa first winding, a second winding and a third winding; and the aplurality of inverter units comprises a first inverter unit, a secondinverter unit and a third inverter unit, and wherein: the first inverterunit and the first winding are connected in series between the powersource and the output filter; the second inverter unit and the secondwinding are connected in series between the power source and the outputfilter; and the third inverter unit and the third winding are connectedin series between the power source and the output filter.
 5. The systemof claim 4, wherein: the first winding of the coupled inductor is woundaround a first leg of a magnetic core; the second winding of the coupledinductor is wound around a second leg of a magnetic core; and the thirdwinding of the coupled inductor is wound around a third leg of amagnetic core.
 6. The system of claim 5, wherein: the first winding, thesecond winding and the third winding of the coupled inductor are woundin a same direction.
 7. The system of claim 5, wherein: the firstwinding, the second winding and the third winding of the coupledinductor are negatively coupled to each other.
 8. A system comprising: afirst inverter connected to a power source; a second inverter connectedto the power source, wherein the second inverter is configured tooperate with a first phase shift from the first inverter; a thirdinverter connected to the power source, wherein the third inverter isconfigured to operate with a second phase shift from the secondinverter; a coupled inductor having a first input, a second input and athird input connected to the first inverter, the second inverter and thethird inverter respectively; and an output filter coupled to an outputof the coupled inductor.
 9. The system of claim 8, wherein the coupledinductor comprises: a magnetic core comprising a first leg, a second legand a third leg; a first winding wound around the first leg; a secondwinding wound around the second leg; and a third winding wound aroundthe third leg, and wherein: outputs of the first winding, the secondwinding and the third winding of the coupled inductor are connectedtogether; and the first winding, the second winding and the thirdwinding of the coupled inductor are negatively coupled to each other.10. The system of claim 9, wherein: the output filter comprises a firstleakage inductance connected in series with the first winding, a secondleakage inductance connected in series with the second winding and athird leakage inductance connected in series with the third winding. 11.The system of claim 8, wherein: the first phase shift and the secondphase shift are configured such that a dominant harmonic of the systemis eliminated as a result of adding the first phase shift and the secondphase shift into the system.
 12. A three-phase inverter systemcomprising: a first phase comprising a plurality of first invertersconnected together through a first coupled inductor; a second phasecomprising a plurality of second inverters connected together through asecond coupled inductor; a third phase comprising a plurality of thirdinverters connected together through a third coupled inductor; and afourth coupled inductor having a first winding connected to the firstcoupled inductor, a second winding connected to the second coupledinductor and a third winding connected to the third coupled inductor.13. The three-phase inverter system of claim 12, wherein: the firstcoupled inductor comprises a plurality of first windings negativelycoupled to each other; input terminals of the plurality of firstwindings are connected to the plurality of first inverters,respectively; and output terminals of the plurality of first windingsare connected together.
 14. The three-phase inverter system of claim 12,wherein: the first winding, the second winding and the third winding ofthe fourth coupled inductor are negatively coupled to each other. 15.The three-phase inverter system of claim 12, wherein: the plurality offirst inverters is configured to operate with a phase shift from oneanother.
 16. The three-phase inverter system of claim 12, wherein: thefirst winding of the fourth coupled inductor is connected between thefirst coupled inductor and a first output filter; the second winding ofthe fourth coupled inductor is connected between the second coupledinductor and a second output filter; and the third winding of the fourthcoupled inductor is connected between the third coupled inductor and athird output filter.